Method and an apparatus for providing channel quality information in a wireless communication system

ABSTRACT

A method for transmitting channel quality information for a downlink channel, the method includes receiving, by a user equipment (UE), configuration information on periodic channel state information (CSI) reporting by higher layer signaling; determining, by the UE, a channel quality information index considering the configuration information on CSI reporting; and transmitting the determined channel quality information index to a base station, wherein the channel quality information index is determined based on a number of available resource element, and wherein an assumption of no resource element allocated for a Channel Status Information-Reference Signal (CSI-RS) is applied when the number of the available resource element is determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.15/048,569 filed on Feb. 19, 2016 (now U.S. Pat. No. 9,722,749 issued onAug. 1, 2017), which is a Continuation of U.S. patent application Ser.No. 14/546,860 filed on Nov. 18, 2014 (now U.S. Pat. No. 9,294,250issued on Mar. 22, 2016), which is a Continuation of U.S. patentapplication Ser. No. 13/509,489 filed on May 11, 2012 (now U.S. Pat. No.8,917,665 issued on Dec. 23, 2014), which is the National Phase of PCTInternational Application No. PCT/KR2011/000216 filed on Jan. 12, 2011,which claims the benefit under 35 U.S.C. § 119(e) to U.S. ProvisionalApplication No. 61/296,007 filed on Jan. 18, 2010, all of which arehereby expressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION Field of the Invention

The following description of the present invention relates to a wirelesscommunication system and, more particularly, to a method and anapparatus for providing channel quality information in a wirelesscommunication system.

Discussion of the Related Art

A Multiple Input Multiple Output (MIMO) system refers to a system thatcan enhance transmission and reception efficiency of data by usingmultiple transmitting antennae and multiple receiving antennae. The MIMOtechnology includes a spatial diversity scheme and a spatialmultiplexing scheme. The spatial diversity scheme may increasetransmission reliability or may widen a cell range through a diversitygain. Thus, the spatial diversity scheme is suitable for datatransmission with respect to a user equipment moving at a high speed.And, by transmitting different data simultaneously, the spatialmultiplexing scheme may increase the data transmission rate withoutincreasing the system bandwidth.

In the MIMO system, each transmitting antenna has an independent datachannel. The transmitting antenna may signify a virtual antenna or aphysical antenna. A reception entity may estimate a channel with respectto each transmitting antenna of a transmission entity, thereby beingcapable of receiving data transmitted from each transmitting antenna.Channel estimation refers to a process of recovering a received signalby compensating for a distortion in a signal, which is caused by fading.Herein, fading refers to an effect wherein the strength of a signal ischanged rapidly due to a multi path-time delay in a wirelesscommunication system environment. In order to perform channelestimation, a reference signal commonly known to the transmission entityand the reception entity is required. A reference signal may also besimply referred to as an RS (Reference Signal) or a Pilot. Also, thereception entity may determine the channel information based upon ameasurement of the received reference signal and may feedback thedetermined channel information to the transmission entity.

A downlink reference signal corresponds to a pilot signal for coherentdemodulation of downlink channel, such as PDSCH (Physical DownlinkShared CHannel), PCFICH (Physical Control Format Indicator CHannel),PHICH (Physical Hybrid Indicator CHannel), PDCCH (Physical DownlinkControl CHannel), and so on. The downlink reference signal may include aCommon Reference Signal (CRS) commonly shared by all user equipmentswithin a cell, and a Dedicated Reference Signal (DRS) specified only fora specific user equipment. The Common Reference Signal (CRS) may also bereferred to as a cell-specific reference signal. And, the DedicatedReference Signal (DRS) may also be referred to as a UE-specificreference signal.

SUMMARY OF THE INVENTION

Discussions on an evolved (or advanced) system (e.g., LTE-Advanced(LTE-A) system) of the conventional 3GPP LTE (Long Term Evolution)system (e.g., a 3GPP LTE release-8 system) are currently under process.Among the reference signals being considered in the LTE-A system, in areference signal for PDSCH demodulation (DeModulation Reference Signal(DMRS)), a number of Resource Elements (REs) being allocated to wirelessresource may be varied in accordance with a channel rank. If channelinformation is calculated without taking into consideration the numberof DMRS REs, which is varied in accordance with the rank, resources maybe wasted or inaccurate channel information may be fed-back.

The present invention proposes a method and an apparatus for providingmore accurate channel quality information, by considering a change inthe number of REs being used by the PDSCH with respect to the rank.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,according to an embodiment of the present invention, a method fortransmitting channel quality information for a downlink channel includesreceiving a downlink signal; calculating a channel quality informationindex for the received downlink signal at least based on a number ofresource elements for a physical downlink shared channel (PDSCH),wherein the number of resource elements for the PDSCH is determined atleast based on an overhead of a demodulation reference signal (DMRS);and transmitting the calculated channel quality information index.

The overhead of the DMRS within one resource block is 12 resourceelements for lower ranks and is 24 resource elements for higher ranks.The lower ranks include rank 1 and rank 2 and the higher ranks includerank 3 to rank 8, or the lower ranks include rank 1 to rank 4 and thehigher ranks include rank 5 to rank 8. Alternatively, the overhead ofthe DMRS within one resource block is 24 resource elements regardless ofdownlink transmission rank.

In case that the downlink is a backhaul downlink, the PDSCH is arelay-PDSCH (R-PDSCH), the overhead of the DMRS within one resourceblock is 6 resource elements for lower ranks, and the overhead of theDMRS within one resource block is 12 resource elements for higher ranks.Further, the lower ranks include rank 1 and rank 2 and the higher ranksinclude rank 3 to rank 8, or wherein the lower ranks include rank 1 torank 4 and the higher ranks include rank 5 to rank 8. Alternatively, theoverhead of the DMRS within one resource block is 12 resource elementsregardless of downlink transmission rank.

According to another embodiment of the present invention, a method forreceiving channel quality information for a downlink channel includestransmitting a downlink signal; receiving a channel quality informationindex for the transmitted downlink signal, wherein the channel qualityinformation index is calculated by the downlink reception entity atleast based on a number of resource elements for a physical downlinkshared channel (PDSCH), and the number of resource elements for thePDSCH is determined at least based on an overhead of a demodulationreference signal (DMRS); and transmitting the downlink signal at leastbased on the channel quality information index.

According to yet another embodiment of the present invention, a userequipment for transmitting channel quality information for a downlinkchannel includes a receiving module configured to receive a downlinksignal from a base station; a transmitting module configured to transmitan uplink signal to the base station; and a processor configured to beconnected to the receiving module and the transmitting module and tocontrol operations of the user equipment, the processor is furtherconfigured to: calculate a channel quality information index for thedownlink signal received through the receiving module at least based ona number of resource elements for a physical downlink shared channel(PDSCH), wherein the number of resource elements for the PDSCH isdetermined at least based on an overhead of a demodulation referencesignal (DMRS), and transmit the calculated channel quality informationindex through the transmitting module.

The overhead of the DMRS within one resource block is 12 resourceelements for lower ranks, and the overhead of the DMRS within oneresource block is 24 resource elements for higher ranks. The lower ranksinclude rank 1 and rank 2 and the higher ranks include rank 3 to rank 8,or the lower ranks include rank 1 to rank 4 and the higher ranks includerank 5 to rank 8. Alternatively, the overhead of the DRMS within oneresource block is 24 resource elements regardless of downlinktransmission rank.

According to yet another embodiment of the present invention, a relaynode for transmitting channel quality information for a backhauldownlink channel includes a receiving module configured to receive abackhaul downlink signal from a base station and to receive an accessuplink signal from a user equipment; a transmitting module configured totransmit a backhaul uplink signal to the base station and to transmit anaccess downlink signal to the user equipment; and a processor configuredto be connected to the receiving module and the transmitting module andto control operations of the relay node, the processor is furtherconfigured to: calculate a channel quality information index for thebackhaul downlink signal received through the receiving module at leastbased on a number of resource elements for a Relay-physical downlinkshared channel (R-PDSCH), wherein the number of resource elements forthe PDSCH is determined at least based on an overhead of a demodulationreference signal (DMRS), and transmit the calculated channel qualityinformation index to the base station through the transmitting module.

The overhead of the DRMS within one resource block is 6 resourceelements for lower ranks, and the overhead of the DRMS within oneresource block is 12 resource elements for higher ranks.

The lower ranks include rank 1 and rank 2 and the higher ranks includerank 3 to rank 8, or the lower ranks include rank 1 to rank 4 and thehigher ranks include rank 5 to rank 8.

Alternatively, the overhead of the DRMS within one resource block is 12resource elements regardless of downlink transmission rank.

According to yet another embodiment of the present invention, a basestation for receiving channel quality information for a downlink channelincludes a receiving module configured to receive an uplink signal froma downlink reception entity; a transmitting module configured totransmit a downlink signal to the downlink reception entity; and aprocessor configured to be connected to the receiving module and thetransmitting module, so as to control operations of the base station,the processor is further configured to: receive a channel qualityinformation index for the downlink signal transmitted through thetransmitting module, wherein the channel quality information index iscalculated by the downlink reception entity at least based on a numberof resource elements for a physical downlink shared channel (PDSCH), andthe number of resource elements for the PDSCH is determined at leastbased on an overhead of a demodulation reference signal (DMRS), andtransmit the downlink signal at least based on the channel qualityinformation index.

The above-mentioned general description of the present invention and theabove-mentioned detailed description of the present invention are merelyexemplary and correspond to an additional description of the appendedclaims of the present invention.

According to each of the above-described embodiments of the presentinvention, by considering the change in the number of REs, which areused in the PDSCH in accordance with the rank, a waste in resource maybe prevented and a method and an apparatus for providing more accuratechannel quality information in a wireless communication system may beprovided.

Additional advantages of the present application will be set forth inpart in the description which follows and in part will become apparentto those having ordinary skill in the art upon examination of thefollowing or may be learned from practice of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block view showing a structure of a transmitterequipped with multi-antennae.

FIG. 2 illustrates a drawing showing a structure of a downlink radioframe.

FIG. 3 illustrates an exemplary drawing of a resource grid with respectto one downlink slot.

FIG. 4 illustrates a drawing showing a structure of a downlinksub-frame.

FIG. 5 illustrates a structural view of a wireless communication systemhaving multi-antennae.

FIG. 6 illustrates a drawing showing a reference signal pattern within adownlink resource block.

FIG. 7 illustrates a drawing for describing a DMRS overhead inaccordance with a respective rank.

FIG. 8 illustrates a drawing showing an example of a periodic channelinformation transmitting method.

FIG. 9 illustrates a drawing showing an example of a method fortransmitting a WB CQI and an SB CQI.

FIG. 10 illustrates a drawing showing an example of a CQI transmissionmethod in case a WB CQI and an SB CQI are transmitted.

FIG. 11 illustrates a drawing for describing an RI transmission method.

FIG. 12 illustrates a general method for calculating a CQI index.

FIG. 13 illustrates a flow chart of an exemplary method for calculatinga CQI index.

FIG. 14 illustrates a wireless communication system including a relay.

FIG. 15 illustrates an example of a Back-haul downlink sub-framestructure.

FIG. 16 illustrates an example of a DMRS pattern in a Back-haul downlinksub-frame structure.

FIG. 17 illustrates a flow chart of a method for calculating andtransmitting a CQI according to an embodiment of the present invention.

FIG. 18 illustrates a structure of a user equipment device, a relaystation device, and a base station device according to a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments described below correspond to predetermined combinationsof elements and features and characteristics of the present invention.Moreover, unless mentioned otherwise, the characteristics of the presentinvention may be considered as optional features of the presentinvention. Herein, each element or characteristic of the presentinvention may also be operated or performed without being combined withother elements or characteristics of the present invention.Alternatively, the embodiment of the present invention may be realizedby combining some of the elements and/or characteristics of the presentinvention. Additionally, the order of operations described according tothe embodiment of the present invention may be varied. Furthermore, partof the configuration or characteristics of any one specific embodimentof the present invention may also be included in another embodiment ofthe present invention, or part of the configuration or characteristicsof any one embodiment of the present invention may replace therespective configuration or characteristics of another embodiment of thepresent invention.

In the description of the present invention, the embodiments of thepresent invention will be described by mainly focusing on the datatransmission and reception relation between the base station and theterminal (or user equipment). Herein, the base station may refer to aterminal node of the network that performs direct communication with theterminal. Occasionally, in the description of the present invention,particular operations of the present invention that are described asbeing performed by the base station may also be performed by an uppernode of the base station.

More specifically, in a network consisting of multiple network nodesincluding the base station, it is apparent that diverse operations thatare performed in order to communicate with the terminal may be performedby the base station or b network nodes other than the base station.Herein, the term ‘Base Station (BS)’ may be replaced by other terms,such as fixed station, Node B, eNode B (eNB), Access Point (AP), and soon. Also, in the description of the present invention, the term basestation may also be used as a term including the concept of a cell orsector. Meanwhile, the term ‘relay’ may be replaced by terms includingRelay Node (RN), Relay Station (RS), and so on. And, the term ‘Terminal’may be replaced by terms including UE (User Equipment), MS (MobileStation), MSS (Mobile Subscriber Station), SS (Subscriber Station), andso on. In this document, the uplink transmission entity may signify auser equipment or a relay station, and the uplink reception entity maysignify a base station or a relay station. And, similarly, the downlinktransmission entity may signify a base station or a relay station, andthe downlink reception entity may signify a user equipment or a relaystation. In other words, an uplink transmission may refer to atransmission from a user equipment to a base station, a transmissionfrom a user equipment to a relay station, or a transmission from a relaystation to a base station. Similarly, a downlink transmission may referto a transmission from a base station to a user equipment, atransmission from a base station to a relay station, or a transmissionfrom a relay station to a user equipment.

The specific terms used in the following description of the presentinvention are provided to facilitate the understanding of the presentinvention. And, therefore, without deviating from the technical scopeand spirit of the present invention, such specific terms may also bevaried and/or replaced by other terms.

In some cases, in order to avoid any ambiguity in the concept of thepresent invention, some of the structures and devices disclosed in thepresent invention may be omitted from the accompanying drawings of thepresent invention, or the present invention may be illustrated in theform of a block view focusing only on the essential features orfunctions of each structure and device. Furthermore, throughout theentire description of the present invention, the same reference numeralswill be used for the same elements of the present invention.

Herein, the embodiments of the present invention may be supported by atleast one the disclosed standard documents for wireless access systemsincluding the IEEE 802 system, the 3GPP LTE system, the LTE-A(LTE-Advanced) system, and the 3GPP2 system. More specifically, amongthe embodiments of the present invention, partial operation steps orstructures of the present invention, which have been omitted from thedescription of the present invention in order to specify and clarify thetechnical scope and spirit of the present invention may also besupported by the above-described standard documents. Furthermore, theterms disclosed in the description of the present invention may bedescribed based upon the above-mentioned standard documents.

The technology described below may be used in a wide range of wirelessaccess systems, such as CDMA (Code Division Multiple Access), FDMA(Frequency Division Multiple Access), TDMA (Time Division MultipleAccess), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA(Single Carrier Frequency Division Multiple Access), and so on. Herein,the CDMA may be realized by a radio technology such as UTRA (UniversalTerrestrial Radio Access) or CDMA2000. The TDMA may be realized by aradio technology such as GSM (Global System for Mobilecommunications)/GPRS (General Packet Radio Service)/EDGE (Enhanced DataRates for GSM Evolution). The OFDMA may be realized by a radiotechnology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, E-UTRA (Evolved UTRA), and so on. The UTRA corresponds to aportion of the UMTS (Universal Mobile Telecommunications System). And,as a portion of the E-UMTS (Evolved UMTS) using the E-UTRA, the 3GPP(3rd Generation Partnership Project) LTE (long term evolution) systemadopts the OFDMA in a downlink and adopts the SC-FDMA in an uplink. TheLTE-A (LTE-Advanced) corresponds to an evolved version of the 3GPP LTEsystem. The WiMAX may be described based upon the IEEE 802.16e standard(WirelessMAN-OFDMA Reference System) and the evolved IEEE 802.16mstandard (WirelessMAN-OFDMA Advanced system). For the clarity in thedescription of the present invention, the present invention will bedescribed based upon the 3GPP LTE system and the 3GPP LTE-A system.Nevertheless, the scope and spirit of the present invention will not belimited only to those of the 3GPP LTE system and the 3GPP LTE-A system.

The structure of a downlink radio frame will now be described withreference to FIG. 1.

In a cellular OFDM wireless packet communication system, uplink/downlinkdata packet transmission is performed in unit of subframe, and onesubframe is defined as a predetermined time duration including aplurality of OFDM symbols. A 3GPP LTE standard supports a type 1 radioframe structure, which can be applied to an FDD (Frequency DivisionDuplex) and also supports a type 2 radio frame structure, which can beapplied to a TDD (Time Division Duplex).

FIG. 1 illustrates the structure of a type 1 radio frame. A downlinkradio frame consists of 10 subframes, and one subframe consists of 2slots in a time domain. The time taken for transmitting one subframe isreferred to as a TTI (transmission time interval), and, for example, thelength of one subframe may be equal to 1 ms, and the length of one slotmay be equal to 0.5 ms. One slot includes a plurality of OFDM symbols ina time domain and includes a plurality of Resource Blocks (RBs) in afrequency domain. Since the 3GPP LTE system uses the OFDMA in adownlink, an OFDM symbol indicates one symbol duration. The OFDM symbolmay also be referred to as an SC-FDMA symbol or a symbol duration. AResource Block (RB) corresponds to a resource allocation unit, and oneResource Block may include a plurality of consecutive subcarriers in oneslot.

The number of OFDM symbols included in one slot may vary depending uponthe configuration of a CP (Cyclic Prefix). The CP may be divided into anextended CP and a normal CP. For example, in case the OFDM symbol isconfigured of a normal CP, the number of OFDM symbols included in oneslot may be equal to 7. And, in case the OFDM symbol is configured of anextended CP, since the length of an OFDM symbol is increased, the numberof OFDM symbols included in one slot is smaller than when the OFDMsymbol is configured of a normal CP. In case of the extended CP, forexample, the number of OFDM symbols included in one slot may be equal to6. In case the user equipment is moving at high speed, or in case thechannel status is unstable, the extended CP may be used in order tofurther reduce the inter-symbol interference.

In case of the usage of a normal CP, since one slot includes 7 OFDMsymbols, one subframe includes 14 OFDM symbols. At this point, the first2 or 3 OFDM symbols of each subframe are allocated to a PDCCH (physicaldownlink control channel), and the remaining OFDM symbols may beallocated to a PDSCH (physical downlink shared channel).

The structure of the radio frame is merely exemplary. And, therefore,the number of subframes included in the radio frame or the number ofslots included in a subframe, and the number of symbols included in oneslot may be diversely varied.

FIG. 2 illustrates an exemplary drawing of a resource grid of a downlinkslot. This corresponds to when the OFDM symbol is configured of a normalCP. Referring to FIG. 2, one downlink slot includes a plurality of OFDMsymbols in the time domain and includes a plurality resource blocks inthe frequency domain. Herein, although it is shown that one downlinkslot includes 7 OFDM symbols in the time domain, and that one resourceblock (RB) includes 12 sub-carriers in the frequency domain, this ismerely exemplary and not limited thereto. Each element within theresource grid is referred to as a resource element (RE). For example,the resource element a(k,l) corresponds to a resource element located inthe k^(th) sub carrier and the l^(th) OFDM symbol. In case of a normalCP, one resource block includes 12×7 resource elements (in case of anextended CP, one resource block includes 12×6 resource elements). Sincethe size of each subcarrier is 15 kHz, one resource block includesapproximately 180 kHz in the frequency domain. N^(DL) corresponds to anumber of resource blocks included in a downlink slot. The value ofN^(DL) may be determined in accordance with a downlink transmissionbandwidth configured by the scheduling of the base station.

FIG. 3 illustrates the structure of a downlink subframe. In onesubframe, a maximum of 3 OFDM symbols located at the front portion of afirst slot within one sub-frame corresponds to a control region whereina control channel is allocated. The remaining OFDM symbols correspond toa data region wherein a Physical Downlink Shared Channel (PDSCH) isassigned. A basic unit of transmission is one subframe. That is, PDCCHand PDSCH are allocated over two slots. Downlink control channels usedin the 3GPP LTE system may include a Physical Control Format IndicatorChannel (PCFICH), a Physical Downlink Control Channel (PDCCH), aPhysical Hybrid automatic repeat request Indicator Channel (PHICH), andso on. The PCFICH is transmitted in the first OFDM symbol of a sub-frameand includes information on the number of OFDM symbols used for thecontrol channel transmission within the sub-frame. The PHICH includesHARQ ACK/NACK signals in response to an uplink transmission. The controlinformation being transmitted through the PDCCH is referred to asDownlink Control Information (DCI). The DCI may include uplink ordownlink scheduling information or may include an uplink transmissionpower control command for a certain terminal (or user equipment) group.The PDCCH may include information on resource allocation andtransmission format of a downlink shared channel (DL-SCH), informationon resource allocation of an uplink shared channel (UL-SCH), paginginformation of a paging channel (PCH), system information of the DL-SCH,resource allocation of a higher layer control message such as a RandomAccess Response transmitted over the PDSCH, a set of transmission powercontrol commands for individual user equipments within the certain userequipment group, transmission power control information, information onthe activation of a Voice over IP (VoIP), and so on. A plurality ofPDCCHs may be transmitted within the control region. And, the userequipment may monitor the plurality of PDCCHs. Herein, the PDCCH may betransmitted in the form of a combination of at least one consecutiveControl Channel Elements (CCEs). A CCE corresponds to a logicalallocation unit used for providing a PDCCH at a coding rate based on awireless channel state. Herein, the CCE corresponds to a plurality ofresource element groups. The formats and the number of available databits of a PDCCH may be decided based upon a correlation between thenumber of CCEs and the coding rate provided by the CCEs. The basestation decides a PDCCH format in accordance with the DCI beingtransmitted to the user equipment and adds a Cyclic Redundancy Check(CRC) to the control information. Depending upon the owner or usage ofthe PDCCH, the CRC may be masked by a Radio Network Temporary Identifier(RNTI). If the PDCCH is for a specific user equipment, a cell-RNTI(C-RNTI) identifier of the user equipment may be masked to the CRC.Alternatively, if the PDCCH is for a paging message, a Paging IndicatorIdentifier (P-RNTI) may be masked to the CRC. If the PDCCH is for asystem information (more specifically, a system information block(SIB)), a system information identifier and a system information RNTI(SI-RNTI) may be masked to the CRC. In order to indicate the randomaccess response, which is a response message to the transmission of arandom access preamble of the user equipment, a random access RNTI(RA-RNTI) may be masked to the CRC.

FIG. 4 illustrates the exemplary structure of an uplink subframe. In afrequency domain, an uplink sub-frame may be divided into a controlregion and a data region. A Physical Uplink Control Channel (PUCCH)including uplink control information is allocated to the control region.And, a Physical uplink shared channel (PUSCH) including user data isallocated to the data region. In order to maintain the single carrierproperty, one user equipment does not transmit the PUCCH and the PUSCHat the same time. The PUCCH for a user equipment is allocated to aresource block pair (RB pair) within a sub-frame. Each of the resourceblocks (RBs) belonging to the RB pair occupies a different sub-carrierfor 2 slots. This state may be referred to as the resource block pair,which is allocated to the PUCCH, as being “frequency-hopped” at the slotboundary.

Modeling of a Multi-Antennae (MIMO) System

A MIMO system is a system that can enhance data transmission andreception efficiency by using multiple transmitting antennae andmultiple receiving antennae. The MIMO technique does not rely on asingle antenna path in order to receive an entire message. Instead, theMIMO technique may combine a plurality of data segments that is receivedthrough a plurality of antennae, thereby receiving the entire data.

FIG. 5 illustrates a block view showing the structure of a wirelesscommunication system having multiple antennae. As shown in FIG. 5(a), ifthe number of transmitting antennae is increased to N_(T), and if thenumber of receiving antennae is increased to N_(R), unlike in the casewherein multiple antennae are used only in the transmitter or thereceiver, a logical channel transmission capacity increases inproportion with the number of antennae. Therefore, the transmission ratemay be enhanced, and the frequency efficiency may be drasticallyenhanced. In accordance with the increase in the channel transmissioncapacity, the transmission rate may be theoretically increased as muchas a value of a maximum transmission rate (R_(o)) using a single antennamultiplied by a rate increase ratio (R_(i)).R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, a MIMO communication system using 4 transmitting antennaeand 4 receiving antennae may theoretically gain a transmission rate 4times greater than that of a single antenna system. After thetheoretical capacity increase of a multi antennae system has been provenin the mid 90s, diverse technologies for realizing a substantialenhancement in the data transmission rate is still under active researchand development. Moreover, some of the technologies are already beingreflected and applied in diverse standards in wireless communication,such as the 3rd generation mobile communications, the next generationwireless LAN, and so on.

Referring to the trend in the many researches on multi antennae up tothe most recent research, research and development on a wide range ofperspectives have been actively carried out, wherein the fields ofresearch include research in the aspect of information theory associatedwith multi antennae communication capacity calculation, research inwireless channel measurement and drawing out models, research intime-spatial signal processing technology for enhancing transmissionreliability and enhancing transmission rate, and so on, in diversechannel environments and multiple access environments.

A communications method in a multi antennae system using mathematicalmodeling will now be described in detail. Herein, it is assumed thatthere are N_(T) number of transmitting antennae and N_(R) number ofreceiving antennae in the system.

Referring to a transmitted signal, when there are N_(T) transmittingantennae, the maximum number of transmittable information is N_(T). Thetransmission information may be expressed as shown below.s=└s ₁ ,s ₂ , . . . s _(N) _(T) ┘^(T)  [Equation 2]

Each of the transmission information s₁, s₂, . . . , s_(N) _(T) may havea different transmission power. When each of the transmission power isreferred to as P₁, P₂ . . . , P_(N) _(T) , the transmission informationwith adjusted respective transmission power may be expressed as shownbelow.{circumflex over (s)}=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T)=[P ₁ s ₁ ,P ₂s ₂ , . . . ,P _(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

Moreover, by using a diagonal matrix P of the transmission power, ŝ maybe expressed as shown below.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Herein, consideration is made on a case wherein N_(T) number of actuallytransmitted signals x₁, x₂, . . . , x_(N) _(T) are configured by havinga weight matrix W applied to an information vector ŝ with adjustedtransmission power. The weight matrix W performs the role of adequatelydistributing transmission information to each antenna in accordance withthe transmission channel status. By using a vector X, x₁, x₂, . . . ,x_(N) _(T) may be expressed as shown below.

$\begin{matrix}{x = {\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}\begin{matrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots\end{matrix} \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Herein, w_(ij) represents a weight between an i^(th) transmittingantenna and a j^(th) information. W may also be referred to as aprecoding matrix.

Meanwhile, different methods may be considered for the transmittedsignal x depending upon 2 different types (e.g., spatial diversity andspatial multiplexing) of the transmitted signal x. In case of spatialmultiplexing, different signals are multiplexed, and the multiplexedsignals are transmitted to the receiving end, so that elements of theinformation vector(s) may have different values. Meanwhile, in case ofspatial diversity, the same signal is repeatedly transmitted through aplurality of channel paths, so that elements of the informationvector(s) may have the same value. Evidently, a combination of spatialmultiplexing and spatial diversity may also be considered. Morespecifically, the same signal may be transmitted through, for example, 3transmitting antennae according to the spatial diversity method, and theremaining signals may be processed with spatial multiplexing, therebybeing transmitted to the receiving end.

When there are N_(R) number of receiving antennae, the received signalsy₁, y₂, . . . , y_(N) _(R) of each of the receiving antennae may beexpressed as a vector as shown below.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

In case of modeling a channel in a multi antennae wireless communicationsystem, a channel may be identified in accordance with a transmittingand receiving antenna index. Herein, a channel passing through receivingantenna i from transmitting antenna j will be expressed as h_(ij). Inh_(ij), it should be noted that, in the index order, the receivingantenna index comes first, and the transmitting antenna index comesnext.

FIG. 5(b) illustrates a channel from N_(T) number of transmittingantennae to receiving antenna i. The channel may be grouped so as to beexpressed in the form of a vector and a matrix. In FIG. 5(b), a channelstarting from a total of N_(T) number of transmitting antennae and beingreceived to receiving antenna i may be expressed as shown below.h _(i) ^(T)=[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Therefore, all channels starting from N_(T) number of transmittingantennae and being received to N_(R) number of receiving antennae may beexpressed as shown below.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

An actual channel passes through a channel matrix H, and an AWGN(Additive White Gaussian Noise) is added. The AWGN n₁, n₂, . . . n_(N)_(R) being added to each of the N_(R) number of receiving antennae maybe expressed as shown below.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

A received signal may be expressed as shown below through theabove-described equation modeling.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\left\lbrack \begin{matrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{matrix} \right\rbrack} + \left\lbrack \begin{matrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{matrix} \right\rbrack} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The number of rows and columns in a channel matrix H indicating thechannel state may be decided by the number of transmitting and receivingantennae. The number of rows in the channel matrix H is equal to thenumber of receiving antennae N_(R), and the number of columns in thechannel matrix H is equal to the number of transmitting antennae N_(T).More specifically, the channel matrix H corresponds to a matrix ofN_(R)×N_(T).

A rank of a matrix is defined as a minimum number among the number ofrows or columns that are independent from one another. Therefore, therank of a matrix cannot be greater than the number of rows or the numberof columns. The rank (rank(H)) of the channel matrix H is limited asshown below.rank(H)≤min(N _(T) ,N _(R))  [Equation 11]

In a MIMO transmission, a ‘Rank’ represents a number of paths that canindependently transmit a signal, and a ‘number of layers’ indicates anumber of signal streams being transmitted through each path. Generally,a transmitting end transmits a number of layers corresponding to thenumber of ranks used in the signal transmission. Therefore, unlessmentioned differently, rank has the same meaning as the number oflayers.

Reference Signal (RS)

In case of transmitting and receiving data by using multi antennae, thechannel status between each transmitting antenna and receiving antennashould be known, so that a correct signal can be received. Therefore, aseparate reference signal should exist for each transmitting antenna.Information for channel estimation and demodulation may be provided by adownlink reference signal (CRS and/or DMRS).

The CRS is used for estimating a channel in a physical antenna end, andcan be commonly received by all user equipments (UEs) within a cell. TheCRS is distributed through the entire bandwidth. The CRS may be used forthe purposes of Channel State Information (CSI) acquisition and datademodulation.

DMRS (or UE-specific reference signal) is used for data demodulation.When performing multi antennae transmission, precoding weight used in aspecific user equipment is directly used for the reference signalwithout modification, and the DMRS enables an equivalent channel to beestimated. The equivalent channel corresponds to a transmitting channelcombined with a precoding weight, which transmitted from each of thetransmitting antennae when the user equipment has received the referencesignal. The conventional 3GPP LTE system (e.g., Release-8) supports amaximum of 4 transmitting antennae transmission, and a DMRS for Rank 1Beamforming is defined. The DMRS for Rank 1 Beamforming is alsoindicated as a reference signal for an antenna port index 5.

FIG. 6 illustrates a pattern wherein a downlink CRS and DMRS are mappedon a downlink resource block. As a unit having a reference signal mappedtherein, a downlink resource block may be expressed as a unit of onesubframe (in time domain)×12 subcarriers (in frequency domain). Morespecifically, in case of a normal CP, one resource block has a length of14 OFDM symbols in the time domain, and, in case of an extended CP, oneresource block has a length of 12 OFDM symbols. FIG. 6 shows a downlinkresource block, when a normal CP is used.

In FIG. 6, resource elements (RE) marked as ‘R0’, ‘R1’, ‘R2’, and ‘R3’indicate CRS positions of antenna port indexes 0, 1, 2, and 3,respectively. Meanwhile, in FIG. 6, a resource element marked as ‘R5’indicates a DMRS position defined in the conventional LTE system (e.g.,LTE Release-8).

Meanwhile, in the LTE-A system, which is an evolved version of the 3GPPLTE system, an extended antenna configuration, MIMO of a high order,multi-cell transmission, evolved MU-MIMO, and so on, are taken intoconsideration. And, in order to operate an efficient reference signaland to support an evolved transmission method, a DMRS-based datademodulation process is also taken into consideration. Morespecifically, apart from a DMRS (R5) for Rank 1 Beamforming, which isdefined in the conventional 3GPP LTE (e.g., 3GPP LTE Release-8), a DMRSfor 2 or more layers may also be defined for supporting datatransmission through an added antenna. It is preferable that such DMRSis set-up so as to exist only in a resource block and layer scheduledfor a downlink transmission by the base station.

An exemplary DMRS pattern that is newly adopted in an LTE Release-9 orLTE-A (LTE Release-10 or subsequent LTE Release) system will now bedescribed in detail with reference to FIG. 6 and FIG. 7. Hereinafter,the LTE Release-9 system and the LTE-A system will be collectivelyreferred to as LTE-A system for simplicity. A DMRS that is used in alower rank in the LTE Release-9/10 DMRS pattern may be positioned in 12REs within a single resource block, and a DMRS that is used in a higherrank may be positioned in 24 REs within a single resource block. Morespecifically, the DMRS pattern shown in FIG. 6 corresponds to anexemplary DMRS pattern for Ranks 1 to 4, and, although the DMRS patternfor Ranks 5 to 8 has the same pattern as the DMRS pattern shown in FIG.6, in case of Ranks 5 to 8 each CDM group may be configured to include 4layers.

In positioning the LTE Release-9/10 DMRS for supporting a maximum ofRank 8 transmission within a radio resource, a DMRS for each layer maybe multiplexed and positioned. Time Division Multiplexing (TDM) refersto positioning a DMRS for 2 or more layers in different time resources(e.g., OFDM symbols). Frequency Division Multiplexing (FDM) refers topositioning a DMRS for 2 or more layers in different frequency resources(e.g., subcarriers). Code Division Multiplexing (CDM) refers tomultiplexing DMRS for 2 or more layers positioned in the same radioresource, by using an orthogonal sequence (or orthogonal covering)across OFDM symbols or across frequency subcarriers for the respectiveRS resource elements. Most particularly, an Orthogonal Cover Code, whichis used for applying the CDM type multiplexing to the RS resourceelements each having an DMRS positioned therein, may be abbreviated toOCC. For example, a Walsh code, a DFT (Discrete Fourier Transform)matrix, and so on may be used as the OCC.

The DMRS pattern of FIG. 6 shows a combination of CDM and FDM. Forexample, CDM Group 1 may be mapped to Ports 1, 2, 5, and 6, and CDMGroup 2 may be mapped to Ports 3, 4, 7, and 8. The number of resourceelements (REs) occupied by the DMRS for each channel rank may vary inaccordance with such mapping relation. And, in case of the CDM+FDMmethod, 12 REs/RB/port may be used in Ranks 1 and 2 (FIG. 7(a)), and 24REs/RB/port may be used in Ranks 3 to 8 (FIG. 7(b)). Alternatively, inaddition to the combined method of the CDM and the FDM, a full CDMmethod may also be taken into consideration. The DMRS pattern of thefull CDM method is identical to that shown in FIG. 6. However, whenmapping the ports, CDM Group 1 may be mapped to Ports 1, 2, 3, and 4,and CDM Group 2 may be mapped to Ports 5, 6, 7, and 8. Accordingly, 12REs/RB/port may be used in Ranks 1 to 4, and 24 REs/RB/port may be usedin Ranks 5 to 8. However, the present invention will not be limited onlyto the above-described example, and, therefore, other adequate DMRSpatterns may be used in accordance the respective transmission rank.

In both of the above-described methods, the number of REs occupied bythe DMRS may be varied depending upon the rank, and 24 REs/RB/port incase of higher rank may have an overhead (or number of REs having theDMRS allocated thereto) 2 times larger than that of a lower rank.

Meanwhile, in order to support a Spectral Efficiency greater than thatof the conventional 3GPP LTE system, the LTE-A system may have anextended antenna configuration. The extended antenna configuration may,for example, be configured of 8 transmitting antennae. A system havingsuch an extended antenna configuration is required to support theoperations of the conventional antenna configuration (i.e., backwardcompatibility). Therefore, the system with extended antennaconfiguration is required to support a reference signal pattern of theconventional antenna configuration, and a new reference signal patternfor an additional antenna configuration is also required. Herein, when aCRS for a new antenna port is added to a system having the conventionalantenna configuration, a disadvantage may occur in that the referencesignal overhead may increase abruptly, thereby decreasing the datatransmission rate. Accordingly, taking such disadvantage intoconsideration, discussions are currently being made on the issue ofdesigning a new reference signal (CSI-RS) for measuring channel stateinformation (CSI) for the new antenna port. Since the CSI-RS does notcorrespond to a signal being transmitted from all subframes, in order toclarify the description of the present invention, the CSI-RS patternwill not be shown in FIGS. 6 and 7.

Transmission of Channel Quality Information

In the 3GPP LTE system, when a downlink reception entity (e.g., userequipment) is connected to (or accesses) a downlink transmission entity(e.g., base station), a measurement such as an RSRP (reference signalreceived power) and an RSRQ (reference signal received quality) for thereference signal transmitted via downlink may be performed at a certaintime. And, the measured result may be reported to the base station on aperiodic basis or on an event triggered basis.

In a cellular OFDM wireless packet communication system, each userequipment reports downlink channel information for each downlink channelstatus via uplink, and the base station may use the downlink channelinformation received from each user equipment, so as to decide atime/frequency resource and Modulation and Coding Scheme (MCS) suitablefor the data transmission for each user equipment.

In case of the conventional 3GPP LTE system (e.g., 3GPP LTE Release-8system), such channel information may include CQI (Channel QualityIndication), PMI (Precoding Matrix Indicator) and RI (Rank Indication).And, depending upon the transmission mode of each user equipment, all orpart of the CQI, PMI, and RI may be transmitted. The CQI may be decidedby a received signal quality of the user equipment, and the receivedsignal quality may generally be decided based upon a measurement of adownlink reference signal. At this point, the CQI value that is actuallybeing delivered to the base station corresponds to an MCS, which canyield a maximum performance while maintaining a Block Error Rate (BLER)to 10% or less in the measured received signal quality.

Also, the reporting method of such channel information may be dividedinto periodic reporting, wherein the channel information is periodicallytransmitted, and aperiodic reporting, wherein the channel information istransmitted in accordance with a request made by the base station.

In case of aperiodic reporting, reporting is set-up for each userequipment by a request bit included in uplink scheduling informationdelivered from the base station to the respective user equipment. And,when each of the user equipments receives this information, therespective user equipment may deliver the channel information, whiletaking into account the respective transmission mode, to the basestation through a physical uplink shared channel (PUSCH).

In case of periodic reporting, a transmission period according to whichchannel information and an offset for the respective transmission periodin subframe units are signaled via a higher layer signal to each userequipment. And, according to the decided transmission period, channelinformation considering the transmission mode of each user equipment maybe delivered to the base station through a physical uplink controlchannel (PUCCH). In case data being transmitted via uplink existsimultaneously in a subframe in which channel information is beingtransmitted in accordance with the decided transmission period, thecorresponding channel information may be transmitted along with datathrough a physical uplink shared channel (PUSCH) instead of beingtransmitted through a physical uplink control channel (PUCCH).

More specifically, the periodic reporting of channel information may befurther divided into 4 reporting modes in accordance with feedback typesof CQI and PMI as shown in Table 1 below.

TABLE 1 PMI Feedback Type PUCCH CQI Feedback Type No PMI Single PMIWideband Mode 1-0 Mode 1-1 (wideband CQI) UE Selected Mode 2-0 Mode 2-1(subband CQI)

Depending upon the CQI feedback type, the reporting method is dividedinto WB (wideband) CQI and SB (subband) CQI, and depending upon the PMItransmission status, the reporting method is divided into No PMI andsingle PMI. Each user equipment may receive information including acombination of a period and an offset of the channel informationtransmission, through RRC signaling from a higher layer. Based upon thereceived information on the channel information transmission period, theuser equipment may transmit channel information to the base station.

FIG. 8 illustrates an example of a method wherein the user equipmentperiodically transmits the channel information. For example, when theuser equipment receives information of a combination of a transmissionperiod of the channel information being equal to ‘5’ and an offset beingequal to ‘1’, the user equipment transmits channel information in 5subframe units. However, given that the 0^(th) subframe is thereferential point, the channel information may be transmitted throughthe PUCCH with 1 subframe offset along an increasing direction of asubframe index. At this point, the index of a subframe may be configuredof a combination of a system frame number (n_(f)) and 20 slot indexes(n_(s), 0˜19) within the system frame. Since one subframe is configuredof 2 slots, a subframe index may be expressed as10×n_(f)+floor(n_(s)/2). A floor(x) function signifies a maximum integerthat is not greater than x.

Depending upon a CQI feedback type, a type transmitting only WB CQI anda type transmitting both WB CQI and SB CQI exist. In case of the typetransmitting only the WB CQI, WB CQI information for the entire band istransmitted at a subframe corresponding to each CQI transmission period.The transmission period of a WB periodic CQI feedback may be set as {2,5, 10, 16, 20, 32, 40, 64, 80, 160} ms or as not transmitted. At thispoint, if the PMI should also be transmitted in accordance with the PMIfeedback type of Table 1, the PMI information is transmitted along withthe CQI information. In case of the type transmitting both WB CQI and SBCQI, the WB CQI and the SB CQI are alternately transmitted.

FIG. 9 illustrates an example of a method of transmitting both WB CQIand SB CQI. Herein, FIG. 9 illustrates a system frequency bandconfigured of, for example, 16 resource blocks (RBs). In case of systemfrequency band having 16 RBs, the frequency band may be configured oftwo BPs (Bandwidth Parts) (BP0 and BP1), and each BP may be configuredof two SBs (subbands) (SB0 and SB1), and each SB may be configured of 4RBs. At this point, the number of BPs and the size of each SB may bedecided depending upon the number of RBs configured in the systemfrequency band, and the number of SBs configuring each BP may be decidedin accordance with the number of RBs, the number of BPs and the size ofeach SB.

In case of the type transmitting both WB CQI and SB CQI, aftertransmitting WB CQI in the CQI transmission subframe, a CQI for the SBhaving a better channel state among SB0 and SB1 within BP0 and the indexof the corresponding SB are transmitted in the next CQI transmissionsubframe, and a CQI for the SB having a better channel state among SB0and SB1 within BP1 and the index of the corresponding SB are transmittedin the following CQI transmission subframe. After transmitting the WBCQI as described above, the CQI information for each BP is sequentiallytransmitted. At this point, CQI information for a BP is sequentiallytransmitted 1˜4 times between the two WB CQIs. For example, when CQIinformation on a BP is transmitted once between the two WB CQIs,transmission may be performed in the order of WB CQI→BP0 CQI→BP1 CQI→WBCQI. In another example, when CQI information on a BP is transmitted 4times between the two WB CQIs, transmission may be performed in theorder of WB CQI→BP0 CQI→BP1 CQI→BP0 CQI→BP1 CQI→BP0 CQI→BP1 CQI→BP0CQI→BP1 CQI→WB CQI. Information on how many times the CQI for a BP is tobe sequentially transmitted between the two WB CQIs is signaled from ahigher layer. And, WB CQI or SB CQI may be transmitted through the PUCCHat a subframe according to the information of a combination of channelinformation transmission period and offset which are signaled from ahigher layer as shown in FIG. 8.

At this point, in case a PMI should also be transmitted in accordancewith the PMI feedback type, the PMI information is transmitted alongwith the CQI information. In this case, if a PUSCH for a uplink datatransmission exists in the corresponding subframe, the CQI and PMI maybe transmitted along with data through the PUSCH instead of the PUCCH.

FIG. 10 illustrates an example of a CQI transmission method when both WBCQI and SB CQI are transmitted. Herein, FIG. 10 shows an exemplarychannel information transmission operation of a user equipment, when acombined information of the channel information transmission periodbeing equal to ‘5’ and of the offset being equal to ‘1’, as shown inFIG. 8, is signaled, and when the information on the BP is sequentiallytransmitted 1 time between the two WB CQIs.

Meanwhile, in case of the transmission of an RI, the RI may be signaledas a combined information including transmission period information ofRI on which multiple of the WB CQI transmission period and informationon an offset of the corresponding transmission period of RI. The offsetin this case is defined as a relative offset with respect to a CQItransmission offset. For example, when the offset of a CQI transmissionperiod is ‘1’, and when the offset of a transmission period of an RI is‘0’, this indicates that the offset of the RI transmission period isidentical to the offset of the CQI transmission period. The offset of RItransmission period may be defined to have a value of 0 or a negativenumber.

FIG. 11 illustrates an exemplary case, wherein the RI transmissionperiod is a multiple of ‘1’ of the WB CQI transmission period, andwherein an offset of the RI transmission period is equal to ‘−1’, when aCQI transmission is set up as shown in FIG. 10. Since the RItransmission period is a multiple of ‘1’ of the WB CQI transmissionperiod, the transmission period of the RI is identical to that of the WBCQI. Also, since the RI offset value ‘−1’ signifies that the value ‘−1’is a relative value for the CQI offset value ‘1’ shown in FIG. 10, theRI may be transmitted while having the subframe index #0 as thereferential point. If the RI offset is equal to ‘0’ instead of ‘−1’, thetransmission subframes of the WB CQI and the RI may overlap one another.And, in this case, the WB CQI may be dropped, so as to transmit the RI.

The CQI, PMI, and RI may be transmitted by the above-describedcombination, and such channel status information may be transmitted fromeach user equipment through RRC signaling of a higher layer. The basestation may take into consideration the channel status of each userequipment and the user equipment distribution status within the basestation, thereby being capable of transmitting adequate information toeach user equipment.

Calculation of Channel Quality Information

When a user equipment calculates a channel quality indicator (CQI)index, it is defined in a 3GPP LTE standard document (e.g., 3GPPTS36.213) that the following assumptions are to be taken intoconsideration.

(1) The first 3 OFDM symbols of a subframe are occupied by controlsignaling.

(2) No resource elements used by primary or secondary synchronizationsignal or a physical broadcast channel (PBCH).

(3) CP length of the non-MBSFN subframes.

(4) Redundancy Version 0.

(5) The PDSCH transmission scheme depending on the transmission modecurrently configured for the UE (which may be the default mode).

(6) The ratio of PDSCH EPRE (Energy Per Resource Element) toCell-specific RS EPRE is as given with the exception for ρ_(A) (ρ_(A)may be assumed to be i) ρ_(A)=P_(A)+Δ_(offset)+10 log₁₀(2)[dB] for anymodulation scheme, if the UE is configured with transmission mode 2 with4 cell-specific antenna ports, or transmission mode 3 with 4cell-specific antenna ports and the associated RI is equal to one; ii)ρ_(A)=P_(A)+Δ_(offset)[dB] for any modulation scheme and any number oflayers, otherwise; The shift Δ_(offset) is given by the parameternomPDSCH-RS-EPRE-Offset which is configured by higher-layer signaling).

The above defined assumptions indicate that the CQI includes informationon channel quality and diverse information on the corresponding userequipment. More specifically, even though the channel quality isidentical, since different CQI indexes may be fed-back in accordancewith the capability of the corresponding user equipment, a certainreference standard is defined.

FIG. 12 illustrates a general CQI index calculation method. As shown inFIG. 12, a user equipment (UE) may receive a reference signal (RS) fromthe base station (eNB) (S1210). The user equipment may determine thestatus of a channel through the received reference signal. Herein, thereference signal may correspond to a common reference signal (CRS) whichis defined in the conventional 3GPP LTE system or may correspond to achannel state information-reference signal (CSI-RS) which is defined ina system having an extended antenna configuration (e.g., 3GPP LTE-Asystem). While satisfying the assumption given for CQI calculation froma channel determined by the user equipment through the reference signal,the user equipment may calculate a CQI index wherein the Block ErrorRate (BLER) does not exceed 10% (S1220). The user equipment may transmitthe calculated CQI index to the base station (S1230). In FIG. 12, theprocess wherein the user equipment determines the status of the channeland obtains an adequate MCS (S1220) may be designed in various methodsin the aspect of implementing the user equipment. For example, the userequipment may use the reference signal so as to calculate a channelstatus or an effective SINR (Signal-to-Interference plus Noise Ratio)(S1221). Based upon the calculated channel status or effective SINR, theuser equipment may derive the highest MCS (S1222). The highest MCSindicates an MCS having a Block Error Rate that does not exceed 10% whenperforming a decoding process, and wherein the MCS satisfies theassumption on the CQI calculation. The user equipment decides a CQIindex associated with the derived MCS and may report the decided CQIindex to the base station (S1223).

In the LTE-A system wherein the standardization process is currently inprogress, discussions are being made on supporting new techniques suchas bandwidth extension, Coordinated Multiple Point (CoMP) transmissionand reception, relay, Multi-User MIMO (MU-MIMO) transmission method fora more enhanced performance. Therefore, while a more complicatedstructure (new reference signal, MU-MIMO, etc.) than that of theconventional LTE system is configured, backward compatibility may alsobe taken into consideration for a co-existence with the conventional LTEsystem. Accordingly, when calculating the CQI, the number of criteriathat are to be considered is increased as compared to the conventionalLTE system.

The present invention proposes a method for calculating a CQI in theLTE-A system and an assumption required for calculating CQI. In short,when considering the DMRS, the adoption of which in the LTE-A iscurrently under discussion, the present invention proposes a methodenabling a CQI suitable for the current channel quality and for theavailable resources to be fed-back to the base station in addition tothe CQI calculation of conventional LTE, by taking into considerationthe size of the DMRS that varies in accordance with the channel rank.

As described above, in the LTE-A, the adoption of a reference signal forPDSCH demodulation (DMRS) and a reference signal for estimating channelstate information (CSI-RI) is currently under discussion, and, herein,the DMRS may have the pattern as FIG. 6. As described above, the numberof REs occupied by the DMRS may vary depending upon the rank, and theDMRS overhead in case of higher rank (24 REs/RB/port occupied) may betwo times larger than that of a lower rank (12 Res/RB/port).

FIG. 13 illustrates a flow chart showing an exemplary method forcalculating a CQI index.

In step S1310, the user equipment may use a signal received from thebase station and determine a best PMI for each rank. For example, theuser equipment may determine the best PMI for rank 1, the best PMI forrank 2, . . . , the best PMI for rank 8, respectively.

In step S1320, the user equipment may decide an SINR for each layerthrough the decided PMI. For example, in case of rank 2, 2 layers mayexist, and the SINR for each of the 2 layers may be decided.

In step S1330, based upon the SINR decided for each layer, the userequipment may decide an SINR for each codeword. This may be decided inaccordance with a codeword-to-layer mapping rule. The codeword-to-layermapping rule may be decided as described below.

At least one or more codewords encoded by the encoder of thetransmitting end may be scrambled by using a UE-specific scramblingsignal. The scrambled codeword may be modulated to complex symbols byusing modulation scheme of BPSK (Binary Phase Shift Keying), QPSK(Quadrature Phase Shift Keying), 16 QAM (Quadrature AmplitudeModulation) or 64QAM in accordance with the type of the transmittedsignal and/or the channel status. Thereafter, the modulated complexsymbols are mapped to one or more layers. If a signal is transmitted byusing a single antenna, one codeword may be directly mapped to one layerand transmitted. However, if a signal is transmitted by using multiantennae, the codeword-to-layer mapping relation may be determined asshown below in Table 2 and Table 3 in accordance with the transmissionscheme.

TABLE 2 Number of code Codeword-to-layer mapping Number of layers wordsi = 0, 1, . . . , M_(symb) ^(layer) −1 1 1 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾ 2 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾ = M_(symb) ⁽¹⁾ x⁽¹⁾(i) = d⁽¹⁾(i) 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 3 2 x⁽⁰⁾(i) =d⁽⁰⁾(i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾ = M_(symb) ⁽¹⁾/2 x⁽¹⁾(i) =d⁽¹⁾(2i) x⁽²⁾(i) = d⁽¹⁾(2i + 1) 4 2 x⁽⁰⁾(i) = d⁽¹⁾(2i) M_(symb) ^(layer)= M_(symb) ⁽⁰⁾/2 = M_(symb) ⁽¹⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) x⁽²⁾(i) =d⁽¹⁾(2i) x⁽³⁾(i) = d⁽¹⁾(2i + 1)

TABLE 3 Number of Number code Codeword-to-layer mapping of layers wordsi = 0, 1, . . . , M_(symb) ^(layer) − 1 2 1 x⁽⁰⁾ (i) = d⁽⁰⁾ (2i) x⁽¹⁾(i) = d⁽⁰⁾ (2i + 1) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 4 1 x⁽⁰⁾ (i) =d⁽⁰⁾ (4i) x⁽¹⁾ (i) = d⁽⁰⁾ (4i + 1) x⁽²⁾ (i) = d⁽⁰⁾ (4i + 2) x⁽³⁾ (i) =d⁽⁰⁾ (4i + 3) $\begin{matrix}{M_{symb}^{layer} = \left\{ \begin{matrix}{M_{symb}^{(0)}/4} & {{{if}\mspace{14mu} M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} = 0} \\{\left( {M_{symb}^{(0)} + 2} \right)/4} & {{{if}\mspace{14mu} M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} \neq 0}\end{matrix} \right.} \\{{{If}\mspace{14mu} M_{symb}^{(0)}\mspace{14mu}{mod}\mspace{14mu} 4} \neq {0\mspace{14mu}{two}\mspace{14mu}{null}\mspace{14mu}{symbols}\mspace{14mu}{shall}\mspace{14mu}{be}}} \\{{appended}\mspace{14mu}{to}\mspace{14mu} d^{(0)}\mspace{14mu}\left( {M_{symb}^{(0)} - 1} \right)}\end{matrix}\quad$

Table 2 shown above corresponds to an exemplary case of signals beingtransmitted by using a Spatial Multiplexing method, and Table 3 shownabove corresponds to an exemplary case of signals being transmitted byusing a Transmit Diversity method. Also, in Table 2 and Table 3,x^((a))(i) indicates an i^(th) symbol of a layer having index a, andd^((a))(i) represents an i^(th) symbol of a codeword having index a. Themapping relation between the number of codewords and the number oflayers used for transmission may be known through the “Number of layers”and the “Number of codewords” shown in Table 2 and Table 3. And, the“Codeword-to-Layer mapping” indicates how the symbols of each codewordare being mapped to the respective layer.

As shown in Table 2 and Table 3, one codeword may be mapped to one layerin symbol units and transmitted. However, as shown in the second case ofTable 3, one codeword may be distributively mapped to a maximum of 4layers. And, when one codeword is distributively mapped to a pluralityof layers, it can be known that the symbols of each codeword can besequentially mapped to each layer and transmitted. Meanwhile, in case ofconfiguring a single codeword-based transmission, one encoder and onemodulating block may exist.

In step S1340, the user equipment may calculate a Spectral Efficiency(SE) best fitting each SINR for each codeword of a respective rankdepending upon the user equipment capability.

In step S1350, the user equipment may calculate a throughput for eachcodeword by multiplying the calculated SE by the number of REs (N_(RE))used for the PDSCH.

In step S1360, the user equipment may calculate a throughput for eachrank by adding the throughputs calculated for each codeword inaccordance with the respective rank.

In step S1370, the user equipment compares the throughput calculated foreach rank and may decide a rank value corresponding to the largestthroughput.

In step S1380, the user equipment may feedback the CQI indexcorresponding to the largest throughput and the corresponding rank tothe base station. Herein, the process of deciding the CQI indexcorresponding to the largest throughput may be performed, for example,by using Table 4 shown below. Table 4 corresponds to an exemplary 4 bitCQI table, which is defined in the 3GPP LTE standard document TS36.213.In Table 4, a throughput obtained by multiplying an efficiency valuedefined for each CQI index by the N_(RE) value is compared with amaximum available throughput in a current channel status calculated bythe user equipment through process steps S1310 to S1370. Thereafter, theCQI index having the most similar value may be determined as the CQIindex that is to be fed-back.

TABLE 4 CQI index modulation code rate × 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

The process steps of S1310 to S1370 for deciding the CQI index aremerely exemplary. Therefore, the present invention will not be limitedonly to the examples given herein. More specifically, depending upon theimplementation of the user equipment, the CQI index value may be decidedby using a variety of methods.

In deciding the CQI index that is to be fed-back as described above, thenumber of REs (N_(RE)) used for the PDSCH is an important factor.However, in the conventional CQI index calculation method, a change inthe number of PDSCH REs was not considered. Therefore, in case the DMRSis adopted, accurate CQI information is required to be fed-backconsidering the fact that the value N varies in accordance with a changein the number of REs to which the DMRS is allocated in accordance withrank. In other words, if it is not taken into account that the number ofPDSCH REs varies in accordance with rank, an inherent error may occurwhen the base station receives a feedback of the CQI index and estimatethe user equipment status, and such inherent error may have a largeinfluence on a subsequent process. More specifically, in case the DMRSoverhead is not considered, CQI index may be determined on theassumption that a larger number of REs is used for the PDSCHtransmission than the actual number of REs that can be used in the PDSCHtransmission. When the base station decides the coding rate of thedownlink data to a high coding rate and transmits the processed databased upon such incorrect CQI information, the number of REs allowingthe user equipment to actually receive the downlink data may be smallerthan the number of REs estimated by the base station. Therefore, thelikelihood of the occurrence of an error is increased high, and, in somecases, it may be impossible to even perform the operation of receivingdownlink data. In order to reduce such error, the present inventionproposes a method that can increase the reliability of a CQI which isshared by the base station and the user equipment, by applying the valueN_(RE) for each rank in the CQI index calculation process consideringthe number of REs of a DMRS which varies in accordance with rank and byselecting the correct CQI index.

Referring back to FIG. 7, the process of allocating the RE for the PDSCHwithin a resource block (RB) considering a DMRS overhead during the CQIindex calculation will now be described in detail. In case of FIG. 7,the assumptions for CQI calculation are made as shown below.

(1) PDCCH is allocated to the first 3 OFDM symbols of one subframe.

(2) The number of REs having the DMRS allocated thereto within oneresource block (the length of one subframe in a time domain×the lengthof 12 subcarriers in a frequency domain) is equal to 12 in case of Ranks1 and 2, and is equal to 24 in case of Ranks 3 to 8.

(3) The CSI-RS and the LTE-A subframe do not exist.

According to the above-described assumptions, in the cases of FIGS. 7(a)and (7)b, REs that can be allocated to PDSCH (data) transmission may bedecided. FIG. 7(a) illustrates a case having a DMRS overhead of a lowerrank (e.g., Ranks 1 to 2), and FIG. 7(b) illustrates a case having aDMRS overhead of a higher rank (e.g., Ranks 3 to 8).

In FIG. 7(a), the DMRS overhead within one resource block is 12REs/RB/port, and an overhead of CRS for 4 transmitting antenna ports is24 REs/RB/port, and the PDCCH occupies 3 OFDM symbols. Accordingly, theRE that is being allocated for the PDSCH (data) transmission correspondsto 104 REs/RB/port.

In FIG. 7(b), the DMRS overhead within one resource block is 24REs/RB/port, and an overhead of CRS for 4 transmitting antenna ports is24 REs/RB/port, and the PDCCH occupies 3 OFDM symbols. Accordingly, theRE that is being allocated for the PDSCH (data) transmission correspondsto 92 REs/RB/port.

As shown in FIG. 7, depending upon the channel rank, there is a largedifference in the number of REs for the PDSCH. Since the difference inthe number of REs for PDSCH may different as much as 12 REs dependingupon the channel rank, if the channel rank is not taken intoconsideration when calculating the CQI index (i.e., if the conventionalCQI index calculation method is applied), this may lead to adisadvantageous result, such as a waste of resource, an increase inerror rate caused by a lack of resource, and so on. Therefore, by usingthe value N_(RE) considering channel rank, when calculating the CQI,unnecessary waste of resource may be prevented and a CQI index bestfitting the transmission scheme may be fed-back.

Also, although it was assumed in the above-described example that, incase of Ranks 1 and 2, the DMRS overhead is 12 REs/RB/port, and, in caseof Ranks 3 to 8, the DMRS overhead is 24 REs/RB/port, the presentinvention will not be limited only to the example given herein. Thepresent invention, for example, just as in the above-described full CDMmethod, in case of Ranks 1 to 4, the DMRS overhead may be 12REs/RB/port, and, in case of Ranks 5 to 8, the DMRS overhead may be 24REs/RB/port. And, even in this case, the best CQI index may becalculated in accordance with the same principle. In other words,according to the present invention, with respect to all of the caseswherein the number of REs (e.g., N_(RE)) having the PDSCH allocatedthereto varies, an optimal CQI index may be calculated and fed-back.

Alternatively, instead of applying a method having the DMRS overheadtaken into consideration for each rank, regardless of the rank, amaximum DMRS overhead (i.e., 24 REs/RB/port) may be considered so as tocalculate the optimal CQI index. Furthermore, in this case, thecomplexity in the CQI calculation may also be simplified.

Wireless Communication System Supporting a Relay Node

Referring to FIG. 14, a relay node (1420) performs the role offorwarding a transmission/reception between a base station (1410) and auser equipment (1431). Herein, a link between the base station (1410)and the relay node (1420) is referred to as a Back-haul link, and thelink between the relay node (1420) and the user equipment(s) (1431) isreferred to as an access link. An uplink receiving function and adownlink transmitting function are required in the base station, and anuplink transmitting function and a downlink receiving function arerequired in the user equipment(s). Meanwhile, a function of performingBack-haul uplink transmission to the base station, a function ofperforming access uplink reception from the user equipment, a functionof performing Back-haul downlink reception from the base station, and afunction of performing access downlink transmission to the userequipment are all required in the relay node.

Meanwhile, the case wherein the Back-haul link is operated in the samefrequency band as the access link is referred to as an ‘in-band’, andthe case wherein the Back-haul link and the access link are eachoperated in a different frequency band is referred to as an ‘out-band’.In case of an in-band relay node, for example, when a back-haul downlinkreception from the base station and an access downlink transmission areperformed at the same time in a predetermined frequency band, atransmitted signal from the transmitting end of the relay node may bereceived by the receiving end of the relay node. And, accordingly, asignal interference or RF jamming may occur at the RF front-end of therelay node. Similarly, when an access uplink reception from the userequipment and a back-haul uplink transmission to the base station isperformed at the same time in a predetermined frequency band, signalinterference may occur at the RS front-end of the relay node. In orderto prevent such signal interference from occurring, the relay node maybe configured that transmission and reception are not performedsimultaneously within the same frequency band. For example, a TDM (TimeDivision Multiplexing) may be used between the Back-haul downlinkreception and the access downlink transmission, so that a Back-hauldownlink can be received by the relay node during a predetermined timeperiod in a predetermined frequency band, and also so that an accessdownlink can be transmitted by the relay node during another timeperiod. Similarly, a TDM may also be used between the Back-haul uplinktransmission and the access uplink reception. Herein, the relay nodethat is operated as described above may also be referred to as ahalf-duplex relay node. In this case, a Guard Time for switching thetransmitting/receiving operations of the relay node is required to beset-up. For example, in order to perform switching between a Back-hauldownlink reception and an access downlink transmission, a Guard Time maybe set-up within a subframe receiving the back-haul downlink.

In a general embodiment of the relay node, within the same frequencycarrier (i.e., within the same IFFT/FFT region) an access link and aback-haul link may be partitioned into subframe units each having thelength of 1 ms by using the TDM method. Herein, the connection with userequipments (hereinafter referred to as ‘legacy user equipments(legacy-UEs)’) operating in accordance with a wireless communicationsystem wherein the relay node is not applied thereto (e.g., theconventional LTE Release-8 or 9 system), is required to be supported. Inother words, backward-compatibility is required to be supported. At thispoint, the relay node is required to support a measuring function of thelegacy user equipments within its own region. Therefore, even in asubframe that is set-up for the back-haul downlink reception, in asection corresponding to the first N (N=1, 2, or 3) number of OFDMsymbols within the subframe, the relay node is required to perform anaccess downlink transmission instead of receiving the Back-hauldownlink.

FIG. 15 illustrates an exemplary Back-haul downlink subframe structure.

In FIG. 15, a relay node non-hearing section (1510) refers to a sectionwherein the relay node transmits an access downlink signal withoutreceiving a Back-haul downlink signal. As described above, this section(1510) may be set-up as 1, 2, or 3 OFDM lengths (the first 1 to 3 OFDMsymbol of a Back-haul downlink subframe).

The guard time (1520) corresponds to a section enabling the relay nodeto switch the transmitting/receiving mode, and the guard time (1530)corresponds to a section enabling the relay node to switch thereceiving/transmitting mode. The length of the guard time may be givenas a value of the time domain, or the length of the guard time may beset-up with k number of time sample values with reference to a timesample (Ts) value. In some cases, the guard time may be set-up as thelength of one or more OFDM symbols. For example, in case a relay nodeBack-haul downlink subframe is consecutively set-up, or in accordancewith a predetermined subframe timing alignment relation, the guard time(1530) of the last portion of the subframe may not be defined or set-up.

In a relay node Back-haul downlink receiving section (1540), the relaynode may receive the PDCCH and PDSCH for the relay node from the basestation. As those physical channels are dedicated to the relay node, thereceived channels may also be expressed as an R-PDCCH (Relay-PDCCH) andan R-PDSCH (Relay-PDSCH).

Meanwhile, the DMRS pattern described in FIG. 6 may be applied to therelay node Back-haul downlink subframe only in a limited situation. Morespecifically, the DMRS pattern of a general subframe shown in FIG. 6 maybe used only in the case wherein the relay node can receive the lastOFDM symbol (the 14^(th) OFDM symbol in case of a normal CP) of theBack-haul downlink subframe. In case the last OFDM symbol of the relaynode Back-haul downlink subframe is set-up as the guard time, the DMRSpattern shown in FIG. 6 cannot be applied to the relay node Back-hauldownlink subframe.

Moreover, a Back-haul downlink transmission, which uses the DMRS for thedemodulation of the R-PDCCH for the relay node, may be configured. Morespecifically, the R-PDDCH may be transmitted through a predeterminedprecoding based rank-1 transmission, spatial multiplexing or a transmitdiversity scheme using the DMRS.

Accordingly, a DMRS pattern may be newly designed for the relay nodeBack-haul link. More specifically, a new DMRS pattern that is differentfrom the DMRS pattern described in FIG. 6 may be applied for the relaynode Back-haul downlink transmission. For example, as shown in FIG. 16,considering the situation wherein one OFDM symbol or a certain number ofOFDM symbols of the last portion of the downlink subframe in the DMRSpattern of FIG. 6 cannot be used for the Back-haul downlink transmission(a situation such as a guard time is set-up), a Back-haul downlinksubframe DMRS pattern format excluding the DMRS REs of the second slot(i.e., DMRS REs being defined in the last two OFDM symbols within thedownlink subframe) may be configured. Evidently, in case a guard time isnot set-up in the relay node Back-haul downlink subframe, the same DMRSpattern as that of FIG. 6 may also be applied to the relay nodeBack-haul downlink.

Even in a case where the DMRS pattern as FIG. 16 is used, in accordancewith the above-described principle of the present invention, a method ofcalculating and transmitting an optimal CQI index considering a changein the number of REs (i.e., N_(RE)) to which the PDSCH can be allocated,wherein the change in the number of REs is caused by a change in theDMRS overhead, may be applied. In this case, the relay node becomes ofdownlink reception entity, and the macro base station becomes thedownlink transmission entity. Thus, a feedback on the CQI index, whichis transmitted from relay node, may be received by the macro basestation.

In the example of FIG. 16, it is assumed that 3 OFDMs are set-up as thePDCCH (or non-hearing section) within one resource block of theBack-haul downlink subframe, and that the R-PDCCH is not set-up, andthat a total of 2 OFDM symbols are used as the guard time.

At this point, in case of a lower rank, the DMRS overhead is 6REs/RB/port, and the number of REs that can be allocated to the datatransmission is equal to 102. Meanwhile, in case of a higher rank, theDMRS overhead is 12 REs/RB/port, and the number of REs that can beallocated to the data transmission is equal to 96. As described above,depending upon the channel rank, there is a large difference is thenumber of REs for the R-PDSCH. Since the number of REs for the R-PDSCHmay be different up to 6 REs depending upon the channel rank, if therank is not taken into consideration in calculation of the CQI index(i.e., if the conventional CQI index calculation method is applied),this may lead to a disadvantageous result, such as a waste of resource,an increase in error rate caused by a lack of resource, and so on.Therefore, by using the value N_(RE), wherein the channel rank is takeninto consideration, when calculating the CQI, unnecessary waste ofresource may be prevented, and a CQI index best fitting the transmissionmethod may be fed-back.

According to the present invention, for all cases wherein the number ofREs (i.e., N_(RE)) that are allocated to a (Back-haul) downlink datatransmission varies in accordance with the channel rank, an optimal CQIindex may be calculated and fed-back.

Alternatively, in the relay node Back-haul downlink, instead of applyinga method having the DMRS overhead taken into consideration for eachrank, regardless of the rank, a maximum DMRS overhead (i.e., 12REs/RB/port) may be considered so as to calculate the optimal CQI index.Furthermore, in this case, the complexity in the CQI calculation mayalso be simplified.

Meanwhile, in case of an access downlink, identical methods forcalculating and feeding-back a CQI index, which considers the DMRSoverhead for a downlink between the above-described base station anduser equipment (macro-UE), may be used between the relay node and theuser equipment (relay-UE).

FIG. 17 illustrates a flow chart showing a method for calculating a CQIaccording to an embodiment of the present invention.

In step S1710, the user equipment may use a signal received from thebase station and determine a best PMI for each rank.

In step S1720, the user equipment may decide an SINR for each layerthrough the decided PMI.

In step S1730, based upon the SINR decided for each layer, the userequipment may decide an SINR for each codeword. This may be decided inaccordance with a codeword-to-layer mapping rule. Table 2 and Table 3shown above correspond to a codeword-to-layer mapping rule when 4transmitting antennae are used. Therefore, in case of an extendedantenna configuration (e.g., 8 transmitting antennae configuration), theSINR for each codeword may be decided in accordance with thecodeword-to-layer mapping rule, which is defined in accordance with theextended antenna configuration.

In step S1740, the user equipment may calculate a Spectral Efficiency(SE) best fitting each SINR for each codeword of a respective rankdepending upon the user equipment capability.

In step 1750, the user equipment takes into consideration a DMRSoverhead, which varies according to rank (in case of a general subframe,the DMRS overhead is 12 REs in a lower rank and 24 REs in a higher rank,and, in case a guard time is set up in the last symbol within the relaynode Back-haul subframe, the DMRS overhead is 6 REs in a lower rank and12 REs in a higher rank), thereby being capable of calculating thenumber of REs (i.e., N_(RE)) that can be allocated for data transmission(PDSCH or R-PDSCH). Also, in order to simplify the CQI calculation, thevalue N_(RE) may be calculated by applying a maximum DMRS overhead (24REs in case of a general subframe, and 12 REs in case of a relay nodeBack-haul subframe), regardless of the rank.

In step S1760, the user equipment may calculate a throughput for eachcodeword by multiplying the SE calculated in step S1740 by the valueN_(RE) calculated in step S1750.

In step S1770, the user equipment may calculate a throughput for eachrank by adding the throughputs calculated for each codeword inaccordance with rank.

In step S1780, the user equipment compares the throughput calculated foreach rank and may decide a rank value corresponding to the largestthroughput.

In step S1790, the user equipment may feedback the CQI indexcorresponding to the largest throughput and the corresponding rank tothe base station. The CQI index corresponding to the largest throughputmay be decided by comparing a throughput value obtained by multiplyingan efficiency value predetermined for each CQI index by the N_(RE)value, with a maximum available throughput in a current channel statuscalculated by the user equipment through process steps S1710 to S1780.Thereafter, the CQI index having the most similar value may be decidedas the CQI index that is to be fed-back.

FIG. 18 illustrates structure of a user equipment device, a relay nodedevice, or a base station device according to a preferred embodiment ofthe present invention. Although the same reference numerals are used forthe user equipment device, the relay node device, or the base stationdevice, this does not signify that each of the devices has the samestructure. More specifically, following describes a separate structureof the user equipment device, the relay node device and the base stationdevice.

The user equipment (UE) device 1800 may include a receiving module 1810,a transmitting module 1820, a processor 1830, and a memory 1840. Thereceiving module 1810 may receive various types of signals, data, andinformation from the base station. The transmitting module 1820 maytransmit various types of signals, data, and information to the basestation. The processor 1830 may be configured to control the overalloperations of the user equipment device 1800, which includes thereceiving module 1810, the transmitting module 1820, the processor 1830,the memory 1840, and an antenna 1850. Herein, the antenna 1850 may beconfigured of a plurality of antennae.

The processor 1830 of the user equipment device may be configured tocalculate a channel quality information index for downlink signalreceived through the receiving module 1810, considering the number ofresource elements (i.e., N_(RE)) for PDSCH transmission determined basedupon an overhead of the DMRS. The processor 1830 of the user equipmentdevice may further be configured to transmit the calculated channelquality information index through the transmitting module 1820.

The overhead of the DMRS within a resource block may be set to 12resource elements in case of lower ranks (e.g., Ranks 1 and 2) and to 24resource elements in case of higher ranks (e.g., Ranks 3 to 8).Alternatively, the overhead of the DMRS within a resource block may beset to 24 resource elements regardless of the downlink transmissionrank.

Various embodiments of the present invention as described above may beidentically applied to details on the user equipment device 1800 and,more particularly, details associated with a configuration realizing theoperations of the processor 1830 of the user equipment device 1800calculating the CQI information.

Additionally, the processor 1830 of the user equipment device mayperform functions of operating and processing information received bythe user equipment device, information that is to be transmitted outsidethe system, and so on. Furthermore, the memory 1840 may store theoperated and processed information for a predetermined period of time.Herein, the memory 1840 may also be replaced by other components such asa buffer (not shown).

Meanwhile, the relay node (RN) device 1800 may include a receivingmodule 1810, a transmitting module 1820, a processor 1830, and a memory1840. The receiving module 1810 may receive various types of signals,data, and information within a Back-haul downlink from the base station,and the receiving module 1810 may also receive various types of signals,data, and information within an access uplink from the user equipment.The transmitting module 1820 may transmit various types of signals,data, and information within a Back-haul downlink to the base station,and the transmitting module 1820 may also transmit various types ofsignals, data, and information within an access uplink to the userequipment. The processor 1830 may be configured to control the overalloperations of the user equipment device 1800, which includes thereceiving module 1810, the transmitting module 1820, the processor 1830,the memory 1840, and an antenna 1850. Herein, the antenna 1850 may beconfigured of a plurality of antennae.

The processor 1830 of the relay node device may be configured tocalculate a channel quality information index for Back-haul downlinksignal received through the receiving module 1810, considering thenumber of resource elements for R-PDSCH transmission determined basedupon an overhead of the DMRS. The processor 1830 of the relay nodedevice may be further configured to transmit the calculated channelquality information index to the base station through the transmittingmodule 1820.

The overhead of the DMRS within one resource block may be set to 6resource elements in case of lower ranks (e.g., Ranks 1 and 2), and to12 resource elements in case of higher ranks (e.g., Ranks 3 to 8).Alternatively, the overhead of the DMRS within a resource block may alsobe set to 12 resource elements regardless of the Back-haul downlinktransmission rank.

Various embodiments of the present invention as described above may beidentically applied to details on the relay node device 1800 and, moreparticularly, details associated with a configuration realizing theoperations of the processor 1830 of the relay node device 1800calculating the CQI information.

Additionally, the processor 1830 of the relay node device may performfunctions of operating and processing information received by the relaynode device, information that is to be transmitted outside the system,and so on. Furthermore, the memory 1840 may store the operated andprocessed information for a predetermined period of time. Herein, thememory 1840 may also be replaced by other components such as a buffer(not shown).

Meanwhile, the base station (eNB) device 1800 may include a receivingmodule 1810, a transmitting module 1820, a processor 1830, a memory1840, and an antenna 1850. The receiving module 1810 may receive varioustypes of signals, data, and information from the user equipment. Thetransmitting module 1820 may transmit various types of signals, data,and information to the user equipment. The processor 1830 may beconfigured to control the overall operations of the user equipmentdevice 1800, which includes the receiving module 1810, the transmittingmodule 1820, the processor 1830, the memory 1840, and an antenna 1850.Herein, the antenna 1850 may be configured of a plurality of antennae.

The processor 1830 of the base station device may be configured toreceive a channel quality information index for a downlink signaltransmitted through the transmitting module 1820. The channel qualityinformation index may be calculated at a downlink reception entity (userequipment or relay node) considering the number of resource elements fora PDSCH (or R-PDSCH) transmission determined based upon an overhead ofthe DMRS. The processor 1830 of the base station device may be furtherconfigured to transmit the downlink signal through the transmittingmodule 1820 considering the channel quality information index.

Various embodiments of the present invention as described above may beidentically applied to details on the base station device 1800 and, moreparticularly, details associated with a configuration realizing theoperations of the processor 1830 of the base station device 1800receiving the CQI information and performing downlink transmission.

Additionally, the processor 1830 of the base station device may performfunctions of operating and processing information received by the basestation device, information that is to be transmitted outside thesystem, and so on. Furthermore, the memory 1840 may store the operatedand processed information for a predetermined period of time. Herein,the memory 1840 may also be replaced by other components such as abuffer (not shown).

The above-described embodiments of the present invention may beimplemented by using a variety of methods. For example, the embodimentsof the present invention may be implemented in the form of hardware,firmware, or software, or in a combination of hardware, firmware, and/orsoftware.

In case of implementing the embodiments of the present invention in theform of hardware, the method according to the embodiments of the presentinvention may be implemented by using at least one of ApplicationSpecific Integrated Circuits (ASICs), Digital Signal Processors (DSPs),Digital Signal Processing Devices (DSPDs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,micro controllers, micro processors, and so on.

In case of implementing the embodiments of the present invention in theform of firmware or software, the method according to the embodiments ofthe present invention may be implemented in the form of a module,procedure, or function performing the above-described functions oroperations. A software code may be stored in a memory unit and driven bya processor. Herein, the memory unit may be located inside or outside ofthe processor, and the memory unit may transmit and receive data to andfrom the processor by using a wide range of methods that have alreadybeen disclosed.

The detailed description of the preferred embodiments of the presentinvention disclosed herein as described above is provided so that thoseskilled in the art can easily implement and realize the presentinvention. Although the embodiment of the present invention has beendescribed with reference to the accompanying drawings, the describedembodiment of the present invention is merely exemplary. Therefore, itwill be apparent to those skilled in the art that various modificationsand variations can be made in the present invention without departingfrom the spirit or scope of the inventions. For example, anyone skilledin the art may combine each component disclosed in the description ofthe embodiments of the present invention. Therefore, it is intended thatthe present invention covers the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents, and it is not intended to limit the present inventiononly to the examples presented herein.

Furthermore, the present invention may be realized in another concreteconfiguration (or formation) without deviating from the scope and spiritof the essential characteristics of the present invention. Therefore, inall aspect, the detailed description of present invention is intended tobe understood and interpreted as an exemplary embodiment of the presentinvention without limitation. The scope of the present invention shallbe decided based upon a reasonable interpretation of the appended claimsof the present invention and shall come within the scope of the appendedclaims and their equivalents. Therefore, it is intended that the presentinvention covers the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents, and it is not intended to limit the present invention onlyto the examples presented herein. Furthermore, claims that do not haveany explicit citations within the scope of the claims of the presentinvention may either be combined to configure another embodiment of thepresent invention, or new claims may be added during the amendment ofthe present invention after the filing for the patent application of thepresent invention.

Although the description of the above-described embodiments of thepresent invention is focused mainly on a 3GPP LTE group system, thepresent invention will not be limited only to the exemplary assumptionmade in the description of the present invention. Herein, theembodiments of the present invention may be used and applied in varioustypes of mobile communication systems having the MIMO technique appliedthereto, by using the same method.

What is claimed is:
 1. A method for transmitting a channel qualityindicator (CQI) to a base station (BS), the method comprising:receiving, by a user equipment (UE), configuration information relatedto periodic channel state information (CSI) reporting via higher layersignaling; determining, by the UE, a CQI index based on theconfiguration information related to the periodic CSI reporting; andtransmitting the determined CQI index to the BS, wherein the CQI indexis determined based on both of a number of physical downlink sharedchannel (PDSCH) resource elements and an assumption that no resourceelement is allocated for a CSI-reference signal (CSI-RS).
 2. The methodof claim 1, wherein the number of the PDSCH resource elements isdetermined based on a UE-specific reference signal overhead, wherein theUE-specific reference signal overhead is determined according to a rankvalue, wherein the UE-specific reference signal overhead within oneresource block for one subframe is 12 resource elements for the rankvalue of 1 or 2, and wherein the UE-specific reference signal overheadwithin one resource block for one subframe is 24 resource elements forthe rank value of 3, 4, 5, 6, 7 or
 8. 3. The method of claim 1, whereinthe UE is configured for reporting at least one of a Precoding MatrixIndicator (PMI) or a Rank Indicator (RI).
 4. The method of claim 1,wherein the configuration information related to the periodic CSIreporting includes a transmission period of the periodic CSI reportingand an offset for the transmission period in subframe units.
 5. Themethod of claim 1, wherein the determined CQI index is transmitted tothe BS on a Physical Uplink Control Channel (PUCCH).
 6. A user equipment(UE) for transmitting a channel quality indicator (CQI) to a basestation (BS), the UE comprising: a receiver configured to receive adownlink signal from the BS; a transmitter configured to transmit anuplink signal to the BS; a processor configured to be connected to thereceiver and transmitter and to control operations of the UE, whereinthe processor is further configured to: control the receiver to receiveconfiguration information related to periodic channel state information(CSI) reporting via higher layer signaling, determine a CQI index basedon the configuration information related to the periodic CSI reporting,and control the transmitter to transmit the determined CQI index, andwherein the CQI index is determined based on both of a number ofphysical downlink shared channel (PDSCH) resource elements and anassumption that no resource element is allocated for a CSI-referencesignal (CSI-RS).
 7. The UE of claim 6, wherein the number of the PDSCHresource elements is determined based on a UE-specific reference signaloverhead, wherein the UE-specific reference signal overhead isdetermined according to a rank value, wherein the UE-specific referencesignal overhead within one resource block for one subframe is 12resource elements for the rank value of 1 or 2, and wherein theUE-specific reference signal overhead within one resource block for onesubframe is 24 resource elements for the rank value of 3, 4, 5, 6, 7 or8.
 8. The UE of claim 6, wherein the UE is configured for reporting atleast one of a Precoding Matrix Indicator (PMI) or a Rank Indicator(RI).
 9. The UE of claim 6, wherein the configuration informationrelated to the periodic CSI reporting includes a transmission period ofthe periodic CSI reporting and an offset for the transmission period insubframe units.
 10. The UE of claim 6, wherein the determined CQI indexis transmitted to the BS on a Physical Uplink Control Channel (PUCCH).11. A method for receiving a channel quality indicator (CQI) from a userequipment (UE), the method comprising: transmitting, by a base station(BS) to the UE, configuration information related to periodic channelstate information (CSI) reporting via higher layer signaling; andreceiving, by the BS, a CQI index, wherein the CQI index is determinedby the UE based on considering the configuration information related tothe periodic CSI reporting, and wherein the CQI index is determinedbased on both of a number of physical downlink shared channel (PDSCH)resource elements and an assumption that no resource element isallocated for a CSI-reference signal (CSI-RS).
 12. A base station (BS)for receiving a channel quality indicator (CQI) from a user equipment(UE), the BS comprising: a receiver configured to receive an uplinksignal from the UE; a transmitter configured to transmit a downlinksignal to the UE; and a processor configured to be connected to thereceiver and the transmitter so as to control operations of the BS,wherein the processor is further configured to: control the transmitterto transmit, to the UE, configuration information related to periodicchannel state information (CSI) reporting via higher layer signaling,and control the receiver to receive a CQI index, wherein the CQI indexis determined by the UE based on the configuration information relatedto the periodic CSI reporting, and wherein the CQI index is determinedbased on both of a number of physical downlink shared channel (PDSCH)resource elements and an assumption that no resource element isallocated for a CSI-reference signal (CSI-RS).